The proposed studies will permit completion of a project to evaluate the significance of variations in platelet size and shape in health and disease. A new centrifugal technique has been devised for estimating the thombocytocrit and thereby mean platelet volume. A refined minicomputer-based Coulter-principle cell sizing system will give independent estimates of platelet MCV, will be used to define the major factors affecting interpretation of Coulter platelet size, and will provide insight into the dynamics of platelet size distribution during stimulated, suppressed, altered and grossly deranged thrombopoiesis. Morphometric analysis of the reversible disc sphere thrombocyte form transformation and platelet activation will yielda geometrico- statistical framework for evaluating measurements of platelet diameters and promises to provide insight into the earliest (rheological) phases of platelet involvement in hemostasis and thrombogenesis.